It means there is no reason. Maybe it is habit, circumstance, or one hasn't given much thought to why one smokes. With the second sentence, it means that "that's just the way things are" and that there probably is no reason, or the reason is unknown. It could also imply there is a reason but the speaker is unwilling to share why.

Answering a question with "just because" is another way of saying "no particular reason" or "that's just the way it is". It's basically a non-answer.

It's sort of saying in a way that it's just 'common sense'

Suppose u don't wanna answer the question directly. In persian it means chonke zira. Suppose sb says that chera cigar mikeshi you would say chonke mikesham dge.